Add CryptoRng for cryptographically secure random number generation

src/random/crypto.rs

@@ -0,0 +1,32 @@
+use std::fs::File;
+use std::io::Read;
+
+use crate::random::Rng;
+
+/// Cryptographically secure RNG sourcing randomness from `/dev/urandom`.
+pub struct CryptoRng {
+    file: File,
+}
+
+impl CryptoRng {
+    /// Open `/dev/urandom` and create a new generator.
+    pub fn new() -> Self {
+        let file = File::open("/dev/urandom").expect("failed to open /dev/urandom");
+        Self { file }
+    }
+}
+
+impl Rng for CryptoRng {
+    fn next_u64(&mut self) -> u64 {
+        let mut buf = [0u8; 8];
+        self.file
+            .read_exact(&mut buf)
+            .expect("failed reading from /dev/urandom");
+        u64::from_ne_bytes(buf)
+    }
+}
+
+/// Convenience constructor for [`CryptoRng`].
+pub fn crypto_rng() -> CryptoRng {
+    CryptoRng::new()
+}